Cellular radiotelephone systems employ radio receivers having very high sensitivities. This high sensitivity is required in radiotelephone receivers to maintain communication throughout a particular coverage area, but such high sensitivity can also result in negative system performance. In analog radiotelephone systems, particular cells are individualized employing a specific supervisory audio tone (SAT) which distinguishes voice communication channels from their surrounding neighbor voice communication channels. Receivers used in these systems detect the SAT modulated on a radio frequency (RF) carrier and couple an audio path within the receivers to land-line telephone networks accordingly. Problems arise, however, when the high sensitivity receivers detect a undesired, low-level RF carrier having a common SAT. These undesired RF carriers might occur in systems where frequency reuse is utilized, intermodulation distortion is high, and adjacent channel interference is present.
Detection of these undesired, low-level RF carriers having a common SAT presents a major problem during handoff in a cellular radiotelephone system. When the receiver receives the undesired signal, the receiver audio path is enabled, allowing barely quieted discriminator noise (due to low received signal strength) to be output to the phone line of the channel. This noise can be summed into a conversation during a channel handoff on the bridge between the source base-station and a target base-station. When the noise is summed into the conversation, an objectionable noise blast occurs.
Thus, a need exists which enables a high sensitivity receiver to complete communication handoff without substantial noise interference.